1. Field of Invention
The present invention relates to an improved pump and actuator for a totally implantable artificial heart.
2. Description of Prior Art
Heretofore a total artificial heart has not been implanted in a human being. As of November 1991 there have been 220 partial artificial hearts implanted. These hearts are partial in the sense that tubes come out of the patient's chest and are connected to a console that provides compressed air to actuate the heart. It is of course a great disadvantage for a patient to be tethered to a large machine. Not only is mobility restricted, but the tubes piercing the patient's chest are a route for infection to enter.
Currently the National Institutes of Health (NIH) has four programs to develop a totally implantable artificial heart that use an electric motor ("Proceedings Cardiovascular Science and Technology Conference," December, 1991, Bethesda Md., published by Association for the Advancement of Medica Instrumentation, Arlington Va.). It is envisioned that the patient will wear a belt of batteries that feed electricity to a coil on the outside of the patients skin. Another coil implanted internally will pick up the electric energy for use by the artificial heart. Thus, there are no wires or tubes that break the skin and provide a dangerous route for infection. A circuit implanted in the patient conditions and controls the electricity that is then passed on the electro-mechanical artificial heart (the pump and actuator). The pump-actuator is the subject of this patent. All of the pump-actuator designs in the NIH programs use electric motors that rotate. Because the pumping action is a linear motion it is necessary to have a mechanism to change the rotary motion to a linear motion. This adds size, weight, and especially complexity to the device. The rotary motor also must have a commutation mechanism, a further complexity.
An extremely important quality of a total artificial heart is its reliability. Repair or adjustment on an implanted heart is difficult and failure is catastrophic. It is a truism that simple machines are more reliable and that complex machines with many parts have greater possibilities for failure. Hence, the complexity of the rotary-to-linear mechanism is an inherent disadvantage with regard to reliability.
Another characteristic of importance is the weight of the artificial heart. The lowest published weight for designs in the NIH program is 671 grams. The designers have noted that these artificial hearts are suitable only for a person who weighs over 150 pounds. For comparison, a diseased natural heart may be 350 grams, an average man's heart 312 grams, and an average woman's heart only 250 grams. A patient may tolerate some extra weight, however, it is apparent from these numbers that it would be a great advantage if future artificial hearts were much lighter.
Linear electric motors are very attractive for artificial hearts as they eliminate the rotary-to-linear motion conversion mechanism. Also, because linear motors have substantially fewer parts, one expects increased reliability over rotary motors and their associated rotary-to-linear mechanisms. Several types of linear motors have been proposed previously for artificial hearts: U.S. Pat. No. 3,842,440 to Karlson (1974); U.S. Pat. No. 3,874,002 to Kurpanek (1975); articles by Yang, C-H, and Nassar, S. A., "A Permanent Magnet Linear Oscillatory Motor for the Total Artificial Heart," Electric Machines and Power Systems, Vol.15, p 381,1988; and by Cathey, J. J., Topmiller, D. A., and Nasar, S. A., "A Tubular Self-Synchronous Motor for Artificial Heart Pump," IEEE Transactions on Biomedical Engineering, BME-33, No. 3, March 1986. None of these proposed linear motors have yielded a practical artificial heart because by using standard design principles they turn out to be too large and heavy.
The efficiency of a linear motor of given output power is dependent upon the size, the bigger the motor the more efficient it is. Thus, to have a linear motor small enough for the application to an artificial heart it must be an inefficient design. This is contrary to instinct and custom. In the present invention the excess heat of a small inefficient motor is transferred to the blood by specific elements incorporated into the design for that purpose.
Depending on how they are categorized there are about five types of linear electric motors. Within each category many variations of internal arrangements for the electrical parts, that is magnets, coils, magnetic circuit material, et. cetera., are possible and are well-known to experts in the field of linear motors. The present invention is not limited to a particular type or configuration of linear motor. Indeed the principles also apply to rotary electric motors. Further objects and advantages of the invention will become apparent from consideration of the drawings and ensuing description.